The present invention relates to a semiconductor memory and, more particularly, to a driving of word lines in a programmable read-only memory (referred hereinafter to as a "PROM") having memory cells each constituted with a field effect transistor having a floating gate.
In such a PROM, a data readout operation is performed by using a difference in threshold voltage between a programmed memory cell and a non-programmed memory cell. The floating gate of the programmed memory cell is charged negative or positive due to electrons or holes injected therein, and therefore, the threshold value thereof takes a high value V.sub.TMW. On the other hand, since the floating gate of the non-programmed memory cell is not charged, its threshold value takes a low value V.sub.TMO. Each memory cell is applied with a voltage between V.sub.TMW and V.sub.TMO as a readout voltage. Therefore, the non-programmed memory cell is turned on while the programmed memory cell is kept non-conductive. Thus, output data is obtained as "1" or "0" dependent upon whether or not the selected memory cell is programmed.
The readout voltage is reduced by a drop of a power supply voltage. When the readout voltage becomes smaller than the low threshold voltage V.sub.TMO due to the drop of the power voltage, all of the memory cells are kept non-conductive regardless of whether or not they are programmed. The low threshold value V.sub.TMO of an N-channel non-programmed memory cell is about 2.5 V, and a readout voltage of at least 3 V is required in order to make the memory cell conductive realiably. Therefore, when the readout voltage is lowered below 3 V due to the power voltage drop, the data readout operation of the EPROM becomes impossible.